The invention relates to a magnetron sputter source.
Magnetron sputter sources of this type have been known for many years and serve for coating substrates in a vacuum. Such magnetron sputter sources are distinguished in that with the aid of a magnetic field a dense plasma is generated in front of the target surface to be sputtered, which permits sputtering the target through ion bombardment at high rates and attaining a layer on the substrate with high growth rate. In such magnetron sputter sources the magnetic field serves as an electron trap which determines significantly the discharge conditions of the gas discharge and plasma confinement. The magnetic field of such a magnetron electron trap is developed such that in the region of the back side of a target to be sputtered closed magnetic pole loops are disposed which do not intersect and, in special cases, form an annular configuration and can also be disposed concentrically, with these magnetic pole loops being disposed antipolar-wise and spaced apart such that field lines close between the poles and herein at least partially penetrate the target where they determine the electron trap effect in the region of the sputter faces. Due to the pole loops disposed one within the other or concentrically, in the target surface region a magnetic field is developed in the form of a tunnel, which forms a closed loop in which the electrons are captured and guided. Based on this characteristic structuring of the magnetron electron trap, an annular plasma discharge is also generated with inhomogeneous plasma density distribution which results in the target likewise being eroded nonuniformly through the nonuniform ion bombardment. In such a magnetron discharge typically an annular erosion trench is generated during operation whereby also problems in the layer thickness distribution on the substrate result and have to be solved. A further disadvantage is that through the developing trench-form erosion pattern of the target the utilization of the target material becomes reduced.
These problems have already been recognized according to DE OS 27 07 144 corresponding to U.S. Pat. No. 5,284,564. The solution proposed is to generate between the loop-form plasma discharge and the target a relative movement such that the plasma sweeps over the target surface. Thereby the erosion profile on the target is to be broadened or flattened and simultaneously the layer distribution on the substrate disposed in front of it to be improved. In the case of rectangular magnetron sputter configurations the magnet system which generates the electron trap is moved, for example according to FIG. 1, back and forth behind the flat target. In the case of round sputter sources, the magnet systems according to FIGS. 22 to 25 is, for example, rotated behind the target about the target axis. Thereby is attained that the plasma loop sweeps over the round target plate. FIGS. 22 and 25 show in addition that the electron trap loop can be shaped differently and can thereby affect the resulting erosion profile.
In configurations in which the substrates are disposed stationarily opposite the magnetron target or rotate about their internal axes in a plane in front of the target, or in which already in the substrate plane over a maximally large area high homogeneity requirements of the coating must be met, special problems are encountered since the distribution and the material utilization problematic must primarily be solved already at the source side and cannot be solved by moving the substrate past such source. Coating installations of this type, in which disk-form substrates are transferred in cycles and positioned in front of a magnetron sputter source and coated there, have greatly gained in significance. In this way today preferably semiconductor wafers are worked or coated for the production of electronic structural components, as well as storage disks for the production of magnetic storage plates and for the production of optical and optomagnetic storage plates.
For coating stationarily disposed disk-form substrates first annular sputter sources were already used before 1980. As stated, through the annular plasma loop a pronounced annular erosion trench is developed in the target, which leads to problems with the layer distribution on the substrate at high precision requirements. Therefore in the case of such source configurations the distance between target and the substrate to be coated must be relatively large, typically must be in the range from 60 to 100 mm. In order to attain good distribution values, in addition the target diameter must be selected to be somewhat greater than the substrate diameter. The relatively large target substrate distance as well as also the relatively large oversizing of the target diameter practically led to the fact that the utilization of the material sputtered off was overall poor. Due to the low economy following as a consequence and the ever increasing distribution requirements made of the coating, round magnetron source configurations with rotating magnet systems were developed, which make possible further improvements in this respect. In order to increase the material utilization and the coating rate it was found that the target substrate distance and the target diameter had to be decreased. But this is only possible if, for one, the plasma confinement takes place such that the plasma extension does not disturb the substrate to be coated and, for another, the target removal is homogeneous over the surface and in particular also in the proximity of the target center is sputtered off.
A first improvement step could be achieved according to a configuration such as is depicted in FIG. 1a. The magnet system 2 is comprised of an outer annular magnet pole 3 and an inner eccentrically offset counter pole 4. The magnet system 2 is supported rotatably about a central rotation axis 6 and is rotated in the rotational direction 7 by a drive, such as with an electromotor, with respect to the stationary target. Due to the eccentric configuration of the inner pole 4, upon application of a discharge voltage on the target 1 an eccentrically rotating plasma loop is generated, which sweeps over a major portion of the target. In FIG. 1b this configuration is shown in cross section, wherein the magnet system 2 is rotatably supported about the source center axis 6 in the rotational direction 7, a substrate s is disposed at a distance d (typically in the range from 40 to 60 mm) from the round target plate 1, with the target 1 being, for example, water-cooled via a cooling device 8. The magnet system 2 is formed of permanent magnets 3, 4 and these are disposed such that the outer pole 3 and the inner pole 4 are spaced apart and antipolar such that the generated field lines B penetrate through the target 1 and form across the target surface the closed tunnel-form magnetic field loop, which forms an electron trap. The return of the permanent magnets takes place across a yoke plate 5 of highly permeable material, such as iron, which is disposed on that side of the permanent magnet poles which is further removed from target 1. To generate an eccentricity of the plasma loop, the inner pole 4 was offset with respect to the rotation axis 6. By choosing this eccentricity the erosion and distribution characteristic can be optimized in a certain range.
A further significant improvement of the magnet system configuration is possible through the completely eccentric formation of the magnetic circuit according to FIGS. 1c and 1d. The width, depicted in FIG. 1c and substantially uniform, of the magnetic tunnel along the entire closed loop permits a more constant and efficient electron trap effect and especially a clearer definition of the eccentricity of the plasma loop, which leads to better results. In FIG. 1d a further embodiment is shown, in which the plasma loop is folded into itself again for example in the form of a type of cardioid curve. Depending on the magnitude of the target and substrate dimensions, a large number of possible loop forms result, such as for example also folded plasma loops, which serve for optimization of the sputter and distribution conditions on the substrate. The advantage of these rotating configurations lies not least therein that the results can be well calculated in advance via the geometric formation alone. Further simulation calculations are possible for the optimization of the design.
Magnetron sputter sources with round planar target and with rotating magnet systems have been marketed for many years by Balzers Aktiengesellschaft in Liechtenstein, for example under the type designation ARQ 125, and are also described in the operating instructions (BB 800 463 BD) for the source in the first edition May 1985.
A further option for affecting the erosion profile comprises shifting the outer magnet pole in the direction of the target sputter face, parallel to the source axis 6, as is shown in FIG. 2. Thereby the field line course B is changed, in particular flattened, such that the erosion profile can be broadened. In such configurations with magnet poles elevated it is also possible to elevate the inner pole 4 in the center if necessary also over the sputter face of the target 1 if the target in the center has an opening provided for this purpose and the provided sputter characteristic permits such. In a stationary coating configuration of substrate s this source formation has the disadvantage that, on the one hand, relatively large target to substrate distances are necessary, the utilization of the sputtered material which arrives on the substrate s is relatively low, since the zones in the outer region, which cannot be utilized, are proportionally large and the target utilization is lower than in rotating systems.
A further and significantly improved formation of a magnetron sputter source configuration for coating disk-form substrates s is depicted in FIG. 3 and described in EP 0 676 791 B1 corresponding to U.S. Pat. No. 5,688,381. This source configuration also has elevated outer poles 3, wherein the pole region itself is preferably developed as a permanent magnet and the magnetic return with respect to the central inner pole 4 takes place across an iron yoke 5. In this source the target body 1 is developed such that it is arched inwardly, thus is concave, and the electron trap is defined such that the hollow volume generated by the inward arching of target 1 forms substantially the plasma discharge volume. It becomes hereby possible to move with the substrate s very close to the target 1, for example 35 mm at a substrate diameter of 120 mm, with the target diameter not being substantially larger than the substrate diameter. Hereby the discharge volume between the concavely developed target 1 and the substrate is substantially delimited by the substrate and the sputter face. This results in the sputtered material being transferred to a very large extent onto the substrate and the margin losses being low. With this source configuration therefore high coating rates at very good economy are possible. Certain restrictions however occur thereby that the control of the erosion profile and of the distribution and the attainment of reproducible conditions, in particular over the target service life, is difficult in this respect. Attempts have therefore been made to affect with additional outer pole configurations 3a, which are disposed between the inner pole 4 and the outer pole 3, the plasma discharge such that at deepened erosion profile a shift of the plasma ring takes place in order to attain a specific compensation effect. At very high required distribution requirements and material utilization degrees this method has, however, certain restrictions.
The present invention therefore has as its object to eliminate the disadvantages of prior art. In particular, the present invention has as its object to realize a magnetron sputter source with elevated outer pole magnet configuration, which combines the advantages of high sputter rates at high degrees of material utilization with very good achievable distribution values on the substrate during the entire target service life, at stable and reproducible conditions. In addition, the magnetron sputter source has high overall economy.
Building on a magnetron sputter source of the above cited type this object is attained through its formation according to the claims.
According to the invention the annular outer pole does not lie in the same plane as the inner pole and, in the margin region of the round target body, is elevated with respect to the inner pole and the rotatable magnet system part receives an inner pole disposed eccentrically to the source axis and receives a second outer pole part between inner pole and static outer pole such that with rotation the tunnel loop of the magnetic field eccentrically sweeps over the sputter face, at high rate and material utilization a good and stable distribution results over the entire target service life which leads to a significant increase of economy. Preferred applications are obtained according to the invention. Preferred further embodiments of the inventive magnetron sputter source are defined in the claims as well.